1. Field of the Invention
The invention is directed to the field of surge arresters to protect high-voltage systems from the effects of over-voltage incidents created by lightening strikes and, more particularly, to the construction of such surge arresters to prevent injury to personnel or equipment due to the generation of fault gases caused by the failure of such surge arresters during over-voltage incidents.
2. Description of the Prior Art
A surge arrester constructed in accordance with the prior art and more particularly one constructed according to one of the embodiments of the device of co-pending U.S. patent application No. 07/658,211 filed Feb. 20, 1991 is shown on the righthand portion of FIG. 1 hereof. (Also see FIGS. 2, 3 and 4) Surge arrester 100 has an elbow-shaped body 102 having a horizontal leg 104 which contains a receptacle 110 to receive a loadbreak bushing insert 148. Loadbreak probe 112 will enter the female contacts (not shown) of insert 148 and insert 148 will be locked in place in receptacle 110 when the annular recess 150 engages detent rib 114 within receptacle 110. Probe 112 engages metal coupling 122 by means of an externally threaded portion (not shown) which engages an internally threaded aperture (not shown) as is well known in the art.
A series of zinc oxide arrester blocks 116 are positioned in the bore of verticle leg 106, and are connected by compression spring 118 and shunt 120 to metal coupling 122. A layer of semi-conductive elastomer 130 such as EPDM rubber containing carbon black is formed about the end of probe 112 and the metal coupling 122. It is desired that the interface between the layer 130 and the probe 112 and metal coupling 122 be as tight as possible so that no gas can pass along such interface.
The blocks 116 are held in place by end cap 128, compression spring 124 and shunt 126. During over-voltage incidents it is possible for one or more of the blocks 116 to fail (represented by the star burst 160 in the center block 116 (see FIG. 2)) and produce large quantities of fault gases (represented by the arrows 162, 164 of FIG. 3.) These gases are intended to propel the blocks 116 downwardly as shown by the arrows 166 (see FIG. 4) after the cap 128 has been forced from verticle leg 106 and thus control the direction of any materials ejected from the failed surge arrester 100 to one which will create the least amount of danger to any person or equipment around the failed surge arrester.
To further seek to insure that the release of fault gases and any other materials ejected is downward and does not occur through the sides of the arrester and to constrain the expansion and possible fracture of the arrester housing, an expansion tube 140 is added to the outside of the housing adjacent verticle leg 106. The expansion tube 140 can be supported by external fins 142 as shown or by a strap over the top of the horizontal leg 104. The expansion tube 140 permits the verticle leg 106 to expand within its elastic limit (see FIG. 3) and allow the gases to be vented downwardly along the outside of blocks 116 (see FIG. 4). The expansion tube 140 also acts to contain any block fragments or other debris which could pierce verticle leg 106 and fly outwardly from surge arrester 100.
However, if the fault gases generated in response to block 116 failure are not fully contained in verticle leg 106, the verticle leg 106 may not expand properly to vent the fault gases to the outside of surge arrester 100 and instead the fault gases may move along the interface of semi-conductor layer 130 into receptacle 110 and force the surge arrester 100 from the loadbreak bushing insert 148 and into persons or equipment in proximity to such insert and leaving the circuit to be protected open.